Method and system for monitoring tissue temperature

ABSTRACT

A thermoacoustic imaging system and method for monitoring tissue temperature within a region of interest, which has an object of interest and a reference that are separated by at least one boundary. The system and method include a thermoacoustic imaging system with an adjustable radio frequency (RF) applicator configured to emit RF energy pulses into the tissue region of interest and heat tissue therein, an acoustic receiver configured to receive bipolar acoustic signals generated in response to heating of tissue in the region of interest, and one or more processors that process at least one received bipolar acoustic signal generated in the region of interest in response to the RF energy pulses to determine a peak-to-peak amplitude thereof and calculate a temperature at the at least one boundary using the peak-to-peak amplitude of the at least one bipolar acoustic signal.

FIELD

The subject disclosure relates to thermoacoustic imaging and inparticular, to a method and system for monitoring tissue temperature.

BACKGROUND

During thermotherapy or cryotherapy, it is necessary to monitortemperature distribution in the tissues to ensure safe deposition ofheat energy in the surrounding healthy tissue and for efficientdestruction of tumor and abnormal cells. To this end, real-timetemperature monitoring with spatial resolution (approximately 1millimeter) and high temperature sensitivity (1 degree Kelvin or better)is needed.

The most accurate temperature monitoring is by directly measuring thetemperature with a thermocouple or thermistor. However, directlymeasuring the temperature is invasive, is generally not preferred, andis simply not feasible.

Several non-invasive temperature monitoring methods have been developed.For example, infrared thermography has been used to monitor tissuetemperature. Although infrared thermography has 0.1 degree Celsiusaccuracy, it is limited to only superficial temperatures.

Ultrasound has been used to monitor tissue temperature. Althoughultrasound has good spatial resolution and high penetration depth, thetemperature sensitivity of ultrasound is low.

Magnetic resonance imaging has been used to monitor tissue temperature.

Although magnetic resonance imaging has advantages of high resolutionand sensitivity, magnetic resonance imaging is expensive, bulky, andslow.

Although techniques for monitoring tissue temperature have beenconsidered, improvements are desired. It is therefore an object at leastto provide a novel method and system for monitoring tissue temperature.

SUMMARY

It should be appreciated that this Summary is provided to introduce aselection of concepts in a simplified form that are further describedbelow in the Detailed Description. This Summary is not intended to beused to limit the scope of the claimed subject matter.

Accordingly, in one aspect there is provided a method for estimatingtissue temperature, the method comprising (i) directing, using a radiofrequency (RF) applicator, one or more RF energy pulses into the tissueregion of interest, the tissue region of interest comprising an objectof interest and at least one reference that are separated by at leastone boundary; (ii) detecting, using an acoustic receiver, at least onebipolar acoustic signal generated in the region of interest in responseto the RF energy pulses and processing the at least one bipolar acousticsignal to determine a peak-to-peak amplitude thereof; and (iii)calculating a temperature at the at least one boundary using thepeak-to-peak amplitude of the at least one bipolar acoustic signal.

In one or more, embodiments, the method comprises estimating atemperature of the object of interest based on the calculatedtemperature at the at least one boundary.

In one or more embodiments, the method comprises detecting, using theacoustic receiver, at least one other bipolar acoustic signal generatedby a second object of interest in response to the RF energy pulses andprocessing the at least one other bipolar acoustic signal to determine apeak-to-peak amplitude thereof.

In one or more embodiments, the method comprises comparing thepeak-to-peak amplitude of the bipolar acoustic signal, the calculatedtemperature at the at least one boundary and the peak-to-peak amplitudeof the other bipolar acoustic signal to estimate a temperature of thesecond object of interest.

In one or more embodiments, the boundary is at a location between atleast two different types of tissue.

In one or more embodiments, the two different types of tissue are one ofmuscle and fat; a blood vessel and fat; and liver tissue and kidneytissue.

In one or more embodiments, the method comprises repeating steps (i) to(iii) during treatment to continuously monitor tissue temperature.

According to another aspect there is provided a system for determiningtissue temperature within a region of interest comprising an object ofinterest and a reference that are separated by at least one boundary,the system comprising a thermoacoustic imaging system comprising anadjustable radio frequency (RF) applicator configured to emit RF energypulses into the tissue region of interest and heat tissue therein and anacoustic receiver configured to receive bipolar acoustic signalsgenerated in response to heating of tissue in the region of interest;and one or more processors configured to: process at least one receivedbipolar acoustic signal generated in the region of interest in responseto the RF energy pulses to determine a peak-to-peak amplitude thereof;and calculating a temperature at the at least one boundary using thepeak-to-peak amplitude of the at least one bipolar acoustic signal.

In one or more embodiments, the one or more processors are furtherconfigured to estimate a temperature of the object of interest based onthe calculated temperature at the at least one boundary.

In one or more embodiments, the one or more processors are furtherconfigured to detect, using the acoustic receiver, at least one otherbipolar acoustic signal generated by a second object of interest inresponse to the RF energy pulses and process the at least one otherbipolar acoustic signal to determine a peak-to-peak amplitude thereof.

In one or more embodiments, the one or more processors are furtherconfigured to compare the peak-to-peak amplitude of the bipolar acousticsignal, the calculated temperature at the at least one boundary and thepeak-to-peak amplitude of the other bipolar acoustic signal to estimatea temperature of the second object of interest.

According to another aspect there is provided a method of monitoringtissue temperature, the method comprising (i) directing, using a radiofrequency (RF) applicator, one or more RF energy pulses into the tissueregion of interest, the tissue region of interest comprising an objectof interest and at least one reference that are separated by at leastone boundary; (ii) detecting, using an acoustic receiver, at least onebipolar acoustic signal generated in the region of interest in responseto the RF energy pulses and processing the at least one bipolar acousticsignal to determine a peak-to-peak amplitude thereof; (iii) calculatinga temperature at the at least one boundary using the peak-to-peakamplitude of the at least one bipolar acoustic signal; and (iv)repeating the directing, detecting and calculating to monitor thetemperature at the at least one boundary.

BRIEF DESCRIPTION OF THE DRAWINGS

embodiments will now be described more fully with reference to theaccompanying drawings in which:

FIG. 1 is a schematic view of an imaging system;

FIG. 2 is a graph showing exemplary bipolar acoustic signals;

FIG. 3 is a graph showing exemplary electric field strength attenuationcurves;

FIG. 4 is a flowchart of a method for estimating tissue temperature;

FIG. 5 is an exemplary region of interest containing an object ofinterest and a reference;

FIG. 6 is a flowchart of another method for estimating tissuetemperature;

FIG. 7 shows various parts of a human body that can be imaged using theimaging system of FIG. 1; and

FIG. 8 is a graph showing how the Grüneisen parameter changes withtemperature.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The foregoing summary, as well as the following detailed description ofcertain examples will be better understood when read in conjunction withthe appended drawings. As used herein, an element or feature introducedin the singular and preceded by the word “a” or “an” should beunderstood as not necessarily excluding the plural of the elements orfeatures. Further, references to “one example” or “one embodiment” arenot intended to be interpreted as excluding the existence of additionalexamples or embodiments that also incorporate the described elements orfeatures. Moreover, unless explicitly stated to the contrary, examplesor embodiments “comprising” or “having” or “including” an element orfeature or a plurality of elements or features having a particularproperty may include additional elements or features not having thatproperty. Also, it will be appreciated that the terms “comprises”,“has”, “includes” means “including but not limited to” and the terms“comprising”, “having” and “including” have equivalent meanings.

As used herein, the term “and/or” can include any and all combinationsof one or more of the associated listed elements or features.

It will be understood that when an element or feature is referred to asbeing “on”, “attached” to, “connected” to, “coupled” with, “contacting”,etc. another element or feature, that element or feature can be directlyon, attached to, connected to, coupled with or contacting the otherelement or feature or intervening elements may also be present. Incontrast, when an element or feature is referred to as being, forexample, “directly on”, “directly attached” to, “directly connected” to,“directly coupled” with or “directly contacting” another element offeature, there are no intervening elements or features present.

It will be understood that spatially relative terms, such as “under”,“below”, “lower”, “over”, “above”, “upper”, “front”, “back” and thelike, may be used herein for ease of description to describe therelationship of an element or feature to another element or feature asillustrated in the figures. The spatially relative terms can however,encompass different orientations in use or operation in addition to theorientations depicted in the figures.

In the following, a method and system for estimating tissue temperatureare described. Generally, the method and system utilize an RF applicatorto obtain thermoacoustic data of tissue within a region of interest(ROI) of a subject. The thermoacoustic data is analyzed to estimatetissue temperature.

Turning now to FIG. 1, an exemplary imaging system is shown and isgenerally identified by reference numeral 20. As can be seen, theimaging system 20 comprises a programmed computing device 22communicatively coupled to an ultrasound imaging system 24 and to athermoacoustic imaging system 26. The ultrasound imaging system 24 andthermoacoustic imaging system 26 are configured to obtain ultrasoundimage data and thermoacoustic image data, respectively, of a tissueregion of interest ROI associated with a subject S.

The programmed computing device 22 in this embodiment is a personalcomputer or other suitable processing device comprising, for example, aprocessing unit comprising one or more processors, system memory(volatile and/or non-volatile memory), other non-removable or removablememory (e.g., a hard disk drive, RAM, ROM, EEPROM, CD-ROM, DVD, flashmemory, etc.) and a system bus coupling the various computer componentsto the processing unit. The computing device 22 may also comprisenetworking capabilities, using Ethernet, Wi-Fi, and/or other suitablenetwork format, to enable connection to shared or remote drives, one ormore networked computers, or other networked devices. One or more inputdevices, such as a mouse and a keyboard (not shown) are coupled to thecomputing device 22 for receiving operator input. A display device (notshown), such as one or more computer screens or monitors, is coupled tothe computing device 22 for displaying one or more generated images thatare based on ultrasound image data received from the ultrasound imagingsystem 24 and/or the thermoacoustic image data received fromthermoacoustic imaging system 26.

The ultrasound imaging system 24 comprises an acoustic receiver in theform of an ultrasound transducer 28 that houses one or more ultrasoundtransducer arrays 30 configured to emit sound waves into the region ofinterest ROI of the subject S. The sound waves directed into the regionof interest ROI of the subject echo off tissue within the region ofinterest ROI, with different tissues reflecting varying degrees ofsound. Echoes that are received by the one or more ultrasound transducerarrays 30 are processed by the ultrasound imaging system 24 before beingcommunicated as ultrasound image data to the computing device 22 forfurther processing and for presentation as ultrasound images that can beinterpreted by an operator. In this embodiment, the ultrasound imagingsystem 24 utilizes B-mode ultrasound imaging techniques assuming anominal speed of sound of 1,540 m/s. As ultrasound imaging systems areknown in the art, further specifics of the ultrasound imaging system 24will not be described further herein.

The thermoacoustic imaging system 26 comprises an acoustic receiver inthe form of a thermoacoustic transducer 32. The thermoacoustictransducer 32 houses one or more thermoacoustic transducer arrays 34 aswell as a radio frequency (RF) applicator 36. It will however beappreciated that the RF applicator 36 may be housed separately from thethermoacoustic transducer 32. The RF applicator 36 is configured to emitshort pulses of RF energy that are directed into tissue within theregion of interest ROI of the subject. In this embodiment, the RFapplicator 36 has a frequency between about 10 Mhz and 100 GHz and has apulse duration between about 0.1 nanoseconds and 10 nanoseconds. The RFenergy pulses delivered to the tissue within the region of interest ROIheat the tissue thereby to induce acoustic pressure waves that aredetected by the thermoacoustic transducer 32. The acoustic pressurewaves that are detected by the thermoacoustic transducer 32 areprocessed and communicated as thermoacoustic image data to the computingdevice 22 for further processing and for presentation as thermoacousticimages that can be interpreted by the operator.

In this embodiment, the ultrasound transducer 28 and thermoacoustictransducer 32 are mechanically interconnected so that the spatialrelationship between the one or more ultrasound transducer arrays 30,the one or more thermoacoustic arrays 34 and the RF applicator 36 areknown. The spatial relationship is set using a centerline of the one ormore ultrasound transducer arrays 34, the one or more thermoacoustictransducer arrays 34, and RF applicator 36. Each centerline is definedas being a mid-point of an area of the respective transduce array.

In this embodiment, the spatial relationship between the one or moreultrasound transducer arrays 30 and the one or more thermoacoustictransducer arrays 34 is such that the centerline of the one or morethermoacoustic transducer arrays 34 is set at know angle a with respectto the centerline (also known as the axial axis or ultrasound transducerarray beam axis) of the one or more ultrasound transducer arrays 30. Thespatial relationship between the one or more thermoacoustic transducerarrays 34 and the RF applicator 36 is such that the centerline of the RFapplicator 36 is spaced-apart and generally parallel to the centerlineof the one or more thermoacoustic transducer arrays 34.

The imaging system 20 utilizes the known spatial relationship betweenthe one or more ultrasound transducer arrays 30 and the one or morethermoacoustic transducer arrays 34 to increase the precision andaccuracy of thermoacoustic imaging.

The coordinate system of the one or more ultrasound transducer arrays 30of the ultrasound transducer 28 and the coordinate system of the one ormore thermoacoustic transducer arrays 34 of the thermoacoustictransducer 32 are mapped by the computing device 22 so that acquiredultrasound and thermoacoustic images can be registered. Alternatively,the thermoacoustic imaging system 26 may make use of the one or moreultrasound transducer arrays 30 of the ultrasound transducer 28 bydisconnecting the one or more ultrasound transducer arrays 30 from theultrasound transducer 28 and connecting the one or more ultrasoundtransducer arrays 30 to the thermoacoustic transducer 32. As will beappreciated, by doing this coordinate mapping between the one or moreultrasound transducer arrays 28 and the one or more thermoacoustictransducer arrays 34 is not required.

During thermoacoustic imaging of a region of interest that includes aboundary between fat or fatty tissue and soft or lean tissue, bipolaracoustic signals are generated that are received by the thermoacoustictransducer 32. This is due to the fact that the soft or lean tissueabsorbs more heat than the fat or fatty tissue causing it to expandrapidly across the boundary and into the fat or fatty tissue, thatexpands less, and then quickly contract. The strength or peak-to-peakvalues of the bipolar acoustic signals depend on the relative absorptionproperties of the fat or fatty tissue and the soft or lean tissue. Thestrength or peak-to-peak values of the bipolar acoustic signals alsodepend on the temperature of the boundary between the fat or fattytissue and soft or lean tissue.

Exemplary bipolar acoustic signals 200, 205 and 210 are shown in FIG. 2.The bipolar acoustic signals 200, 205 and 210 are generated in responseto thermoacoustic imaging of a tissue region of interest ROI comprisinga first tissue 220 and a different type of second tissue 225 that areseparated by a boundary 215. The dashed line 230 indicates a time pointcorresponding to the boundary 215. The differences in the peak-to-peakvalues of the bipolar acoustic signals 200, 205 and 210 represent theextent to which the first tissue 220 expands into the boundary 215 andinto the second tissue 225 before contracting. As the temperature of theboundary 215 increases, the amount that the first tissue 220 expandsinto the boundary 215 and into the second tissue 225 increases. As such,the peak-to-peak amplitude of each bipolar acoustic signal 200, 205 and210 is proportional to a temperature at the boundary 215. As can beseen, the peak-to-peak value of bipolar acoustic signal 200 is greaterthan that of bipolar acoustic signals 205, 210 and the peak-to-peakvalue of bipolar acoustic signal 205 is greater than that of bipolaracoustic signal 210. As such, the temperature of the boundary 215 whenbipolar acoustic signal 200 is generated is greater than the temperatureof the boundary 215 when bipolar signal 205 is generated. Similarly, thetemperature of the boundary 215 when bipolar acoustic signal 205 isgenerated is greater than the temperature of the boundary 215 whenbipolar signal 210 is generated.

Exemplary electric field strength attenuation curves 300 and 305 areshown in FIG. 3. Each electric field strength attenuation curve 300, 305represents the electric field strength attenuation of tissue 310, 315,respectively, as a function of distance from the RF applicator 36 of thethermoacoustic imaging system 26. The tissue 310 associated withelectric field strength attenuation curve 300 has a differenttemperature and Grüneisen parameter than the tissue 315 associated withelectric field strength attenuation curve 305. As will be appreciated,temperature has either a positive or negative effect on thermoacousticbipolar signals depending on the temperature relationship betweenelectric field strength and Grüneisen parameter for a respective type oftissue.

The imaging system 20 exploits the relationship between thermoacousticbipolar signals and temperature to estimate tissue temperature duringthermoacoustic imaging.

Turning now to FIG. 4, a method of determining tissue temperature duringthermoacoustic imaging is shown. Initially during the method, a regionof interest ROI within the subject S to be imaged that contains anobject of interest and a reference separated by at least one boundary islocated (step 410). In this embodiment, the region of interest ROI islocated using the ultrasound imaging system 24. Specifically, ultrasoundimage data obtained by the ultrasound imaging system 24 is communicatedto the computing device 22. The ultrasound image data is processed bythe computing device 22 and a reconstructed ultrasound image ispresented on the display device. The operator moves the ultrasoundtransducer 28 on the subject's body until the region of interest islocated. When locating the region of interest, the computing device 22overlays information associated with the angle of the centerline of theone or more transducer arrays 30 of the ultrasound transducer 28 overtopof the reconstructed ultrasound image an the display device. Theinformation is used to provide feedback to the operator to ensure theaxial axis of the ultrasound transducer 28 is generally perpendicular toa boundary between the object of interest and the reference.

An exemplary region of interest 500 containing an object of interest 505and a reference 510 is shown in FIG. 5. In this embodiment, the objectof interest 505 is the subject's liver and the reference 510 is thesubject's kidney. Also shown in FIG. 5 is the RF applicator 36 and thethermoacoustic transducer 32.

At least one boundary between the object of interest and the referenceis then identified in the reconstructed ultrasound image (step 420). Inthis embodiment, the at least one boundary is identified by the operatorusing an input device such as a mouse coupled to the computing device22. Specifically, the operator draws a box that encompasses at least aportion of the object of interest 505, at least a portion of thereference 510 and the identified boundary between the portions of theobject of interest and the reference. The computing device 22 providesfeedback to the operator via the display device to indicate theapproximate angle between the box and the boundary to ensure the box isgenerally perpendicular to the boundary.

An exemplary box 515 is shown in FIG. 5. As can be seen, the box 515encompasses a portion of the object of interest 505 (the liver), aportion of the reference 510 (the kidney), and the boundary 520 betweenthe object of interest 505 and the reference 510. The boundary 520 isselected at a particular location 525 where the liver and the kidney arein close relation to one another.

The RF applicator 36 is conditioned to generate short RF energy pulses.The RF energy pulses are directed into the region of interest 500 todeliver energy to the object of interest 505 and the reference 510within the region of interest ROI. In response, bipolar acoustic signalsare generated that are detected by the thermoacoustic transducer 32(step 430).

Since the angle α between the centerline of the one or more transducerarrays 30 of the ultrasound transducer 28 and the centerline of the oneor more transducer arrays 34 of the thermoacoustic transducer 32 isknown, the operator is able to adjust position of the thermoacoustictransducer 32 with respect to the subject's body such that thethermoacoustic imaging system 26 is able to obtain thermoacoustic imagedata of the region of interest at a desired imaging angle σ. The desiredimaging angle σ is such that the centerline of the one or moretransducer arrays 34 of the thermoacoustic transducer 32 extends throughthe boundary 520 between the object of interest 505 and the reference510.

The bipolar acoustic signals are in turn communicated to the computingdevice 22 for processing (step 440). In this embodiment, the computingdevice 22 is programmed to determine the peak-to-peak amplitudes of thebipolar acoustic signals.

The temperature of the boundary 520 is determined using the peak-to-peakamplitudes of the bipolar acoustic signals (step 450). In thisembodiment, the temperature of the boundary 520 is determined based onthe following.

The pressure rise of the acoustic pressure wave is proportional to adimensionless parameter called a Grüneisen parameter and local fluence.The local fluence is dependent on tissue parameters such as for examplethe absorption coefficient, scattering coefficient and anisotropyfactor. The local fluence does not change significantly withtemperature.

The Grüneisen parameter is dependent on isothermal compressibility, thethermal coefficient of volume expansion, the mass density, and thespecific heat capacity at constant volume of the tissue. As shown inFIG. 8, the Grüneisen parameter changes significantly with temperature.As such, the strength or peak-to-peak value of the bipolar acousticsignal generated from the acoustic pressure wave is dependent on tissuetemperature.

Using the Grüneisen parameter and strength or peak-to-peak value of thebipolar acoustic signal, the temperature of the boundary can bedetermined. In this embodiment, the temperature of the boundary iscalculated using the following equations.

The thermoacoustic pressure p(r,t) produced by a heat source H(r, t)obeys the following equation:

$\begin{matrix}{{{\nabla^{2}{p\left( {\underset{\_}{r},t} \right)}} - {\frac{1}{c^{2}}\frac{\partial^{2}}{\partial t^{2}}{p\left( {\underset{\_}{r},t} \right)}}} = {{- \frac{\beta}{C_{p}}}\frac{\partial\;}{\partial t}{H\left( {\underset{\_}{r},t} \right)}}} & (1)\end{matrix}$

where r is the spatial position vector, β is the isobaric volumeexpansion coefficient, c is the sound speed and C_(p) is the specificheat capacity. Solving equation 1 with respect to the acoustic pressurewave p(r, t) yields the following forward problem:

$\begin{matrix}{{p\left( {\underset{\_}{r},t} \right)} = {{\frac{\beta}{4\; \pi \; C_{p}}{\int{\int{\int{\frac{\partial\; \underset{\_}{r}}{{\underset{\_}{r} - {\underset{\_}{r}}^{\prime}}}\frac{\partial{H\left( {{\underset{\_}{r}}^{\prime},t^{\prime}} \right)}}{\partial t^{\prime}}}}}}}_{t^{\prime} = {t - \frac{r^{\prime}}{C}}}}} & (2)\end{matrix}$

The heat source H(r, t) is modeled as the product of two factors whichare the spatial distribution of energy absorption A(r) and the temporalirradiation function I(t)

The spatial distribution of energy absorption A(r) is determined basedon characteristics of the tissue(s) being imaged. Since thethermoacoustic transducer array 30 has a finite bandwidth, receivedthermoacoustic data p_(d)(r,t) is a result of the convolution ofacoustic pressure wave p(r, t) and the impulse response of thethermoacoustic transducer array 30 h(t) as set out in equation 3:

p _(d)( r ,t)=p( r ,t)*_(t) h(t)   (3)

where *_(t) denotes a one-dimensional temporal convolution.

As will be appreciated, for conventional thermoacoustic imaging, thegoal is to recover the spatial absorption distribution A(r) by invertingthe forward problem. As such, the irradiation function is modeled as atemporal function that is uniform throughout the field at a given timepoint.

Due to the limited bandwidth of the thermoacoustic transducer array usedto receive thermoacoustic data, accurately recovering the absorptiondistribution is not trivial. As such, extracting quantitativeinformation such as tissue temperature from thermoacoustic data requiressophisticated methods beyond that of conventional reconstructionmethods.

When the region of interest is heated with a pulse of RF energy, thepower deposition per unit volume A(r) is expressed as:

A( r )=ωϵ₀ϵ″_(r) E ²( r )   (4)

where ω is the radian frequency, ϵ₀ is the vacuum permittivity, E″_(r)is the imaginary part of the relative permittivity of the tissue andE(r) is the electric field strength. The strength of thermoacoustic dataS(r) obtained from a tissue is the product of the deposited energy andthe Grüneisen parameter of the tissue Γ:

S( r )=ΓA( r )=Γωϵ₀ϵ′_(r) E ²( r )   (5)

Within dielectric lossy medium, the electric field strength isattenuated as it propagates through the medium. The amount ofattenuation is determined by various factors such as for examplecharacteristics of region of interest and characteristics of the RFapplicator 36. The spatial distribution of the electric field is:

E( r )=E ₀ E _(A)( r )   (6)

where E₀ is the maximum electric field strength forward to the region ofinterest and E_(A)(r) is the attenuation of the electric field over agiven space. For a simple 1D case, the attenuation E_(A)(r) can beexpressed in exponential form:

E _(A)(d)=e ^(−ηd)   (7)

where η is the electric field absorption coefficient of the region ofinterest and d is the distance of the region of interest from theapplicator.

As mentioned, the strength or peak-to-peak amplitudes of the bipolarsignals represent the absorption property difference between the objectof interest and the reference. Further, the phase of the thermoacousticdata at the boundary indicates which tissue (object of interest or thereference) has a higher or lower absorption coefficient. The strength orpeak-to-peak amplitudes S_(l) of the thermoacoustic signals measured atthe boundary location, is expressed in equation 8.0:

S _(l)=μ(Γ₁ϵ″_(r,1)−Γ₂ϵ′_(r,2))ωϵ₀ E _(l) ²   (8.0)

where subscripts 1 and 2 denote two different tissues located on eachside of the boundary, r_(l), and E_(l) denotes the incident electricfield strength at the boundary, and μ is a calibration scaling constant.The calibration scaling constant of the given system will depend onvarious factors such as transducer characteristics and applied signalprocessing techniques. Without loss of generality, the value of thecalibration scaling constant can assumed to be 1 for the derivationsbelow.

As shown in equation 8.0, the strength of the acquired thermoacousticdata is determined by tissue properties and the strength of the electricfield.

Since the properties of the reference are known, to determine thetemperature of the object of interest, only the strength of the electricfield at the boundary is required. Put another way, since tissue with ahigher temperature will have different dielectric and thermal propertiesthan lower temperature tissue, the temperature at the boundary betweenthe reference and the object of interest is deduced.

The Grüneisen parameter of the tissue Γ is a function of temperature. Inthis embodiment, it is assumed that the relationship between theGrüneisen parameter of the tissue Γ and temperature of the tissue islinear:

Γ(T)=aT+b   (8.1)

Where a is the slope of the linear relationship, T is an offset of thetemperature which applies to a linear portion of the modeled temperaturerange (for example corresponding to a linear temperature range of 20 to80 degrees Celsius), and b is the value at T=0. It will be appreciatedthat in other embodiments higher order functions or other types ofmathematical relationships can be used.

Using the linear relationship of equation 8.1, equation 8.0 is rewrittenas:

S _(l)=((a ₁ T+b ₁)ϵ″_(r,1)−(a ₂ T+b ₂)ϵ″_(r,2))ωϵ₀ E _(l) ²   (8.2)

Solving for temperature T generates equation 8.3:

T=(((S _(l)/(ωϵ₀ E _(l) ²))−b ₁ϵ″_(r,1) −b ₂ϵ″_(r,2))/(a ₁ϵ″_(r,1) +a₂ϵ″_(r,2))   (8.3)

As will be appreciated, the dielectric properties of tissue are alsotemperature dependent. For each tissue type the relationship between thedielectric properties and temperature are unique. The above equationsassume that the dielectric properties of tissue are fixed. To moreaccurately determine temperature, the permittivity of each tissue typeis modeled as a function of temperature. As a result, equation (8.2) isrewritten as:

S _(l)=(Γ₁(T)ϵ″_(r,1)(T)−Γ₂(T)ϵ″_(r,2)(T))ωϵ₀ E _(l) ²   (8.4)

or

S _(l)=((a ₁ T+b ₁)ϵ″_(r,1)(T)−(a ₂ T+b ₂)ϵ″_(r,2)(T))ωϵ₀ E _(l) ²  (8.5)

By rearranging terms, the following function of temperature is defined:

g(T)≡((a ₁ T+b ₁)ϵ″_(r,1)(T)−(a ₂ T+b ₂)ϵ″_(r,2)(T))=S _(l)/(ωϵ₀ E _(l)²)   (8.6)

As such, for different temperatures, the function g(T) is tabulated frommodels and experiments.

As will be appreciated, the incident electric field E_(l) at theboundary can be determined using Eq. (6):

E _(l) =E ₀ e ^(−η) ^(ref) ^(d) ^(ref)   (9.0)

where E₀ is the electric field strength at the start of the reference,η_(ref) is, the attenuation coefficient of the reference, d_(ref) is thethickness of the reference. In this embodiment, the electric fieldstrength, E₀ is modeled via a finite-difference time domain (FDTD)method and is inferred based on measurements at the skin.

Electric field attenuation of tissue also depends on temperature. Assuch, equation 9 can be rewritten with temperature dependence asfollows:

E _(l)(T)=E ₀ e ^(−η) ^(ref) ^((T)d) ^(ref)   (9.1)

Using the above equations, the following function of temperature isdefined:

h(T)≡((a ₁ T+b ₁)ϵ″_(r,1)(T)−(a ₂ T+b ₂)ϵ″_(r,2)(T))e ^(2η) ^(ref)^((T)) =S _(l)/(ωϵ₀ E ₀ ² e ^(−2d) ^(ref) )   (9.2)

For different temperatures, the function h(T) may be tabulated frommodels and experiments. Each component of h(T) is either measured ormodeled for different temperatures. Using the function h(T) thetemperature at the boundary is determined.

It is assumed that the object of interest and reference have the sametemperature at the boundary. As such, once the temperature of theboundary 520 is determined, the temperature of the object of interestand the reference are estimated. Steps 430 to 450 may be repeated tocontinuously monitor the temperature of the object of interest duringmedical procedures such as for example during hyperthermia treatment.

In another embodiment, the temperature of the boundary, object ofinterest and reference may be used to estimate the'temperature of asecond object of interest (which in a separate embodiment, cancorrespond to the entire region of interest or an area affected by amedical procedure). An exemplary method 600 is shown in FIG. 6. Themethod follows step 450 described above, wherein the temperature of theboundary, object of interest and reference are determined (step 610). Asecond object of interest is located using the ultrasound imaging system24 (step 620). In this embodiment, the second object of interest is atumour. RF energy pulses are directed to deliver energy to the secondobject of interest. In response, bipolar acoustic signals are generatedthat are detected by the thermoacoustic transducer 32 (step 630). Thebipolar acoustic signals are communicated to the computing device 22 forprocessing (step 640). In this embodiment, the computing device 22 isprogrammed to determine the peak-to-peak amplitudes of the bipolaracoustic signals.

By comparing the peak-to-peak amplitudes of the bipolar acoustic signalsdetermined during step 440, the temperature of the boundary, object ofinterest and reference determined during step 450, and the peak-to-peakamplitudes of the bipolar acoustic signals determined during step 640,the temperature of the second object of interest may be estimated (step650). Steps 610 to 650 may be repeated to continuously monitor thetemperature of the second object of interest during medical proceduressuch as for example during hyperthermia treatment.

Although in embodiments the object of interest is described as being theliver and the reference is described as being the kidney, those skilledin the art will appreciate that thermoacoustic data may be obtained forother parts of the body. As shown in FIG. 7, various parts of the bodythat may be imaged using the above-described system and method includethe epi/pericardial adipose tissue 1301, the liver 1302, subcutaneousadipose tissue 1303, visceral adipose tissue 1304, subcutaneousgluteal-femoral adipose tissue 1305, perivascular adipose tissue 1306,myocardial fat 1307, pancreas fat 1308, renal sinus fat 1309, and musclefat 1310.

It will be appreciated that in some embodiments the thermoacoustic datamay be corrected according to an thermoacoustic data adjustment. Forexample, thermoacoustic 201 signals propagate through space in the formof acoustic pressure waves. Received signals at the ultrasoundtransducer array can be expressed according to equation 10:

p _(S)(t)=∫_(S) p( r ,t)dS   (10)

where S is the surface area of the ultrasound transducer array. Both theproperties of the thermoacoustic transducer array and its positioningrelative to the region of interest may have an effect on thethermoacoustic data.

Although in embodiments described above the reference is described asbeing selected by the operator, those skilled in the art will appreciatethat alternatives are available. For example, in another embodiment thereference may be automatically defined, using an algorithm performed bythe computing device based on known geometry and/or known ultrasoundproperties of particular types of tissue within the region of interest.

Further, the boundary between the reference and the object of interestmay be automatically defined using algorithms based on ultrasoundsegmentation or thermoacoustic data analysis. As will be appreciated,both operator-defined and automatic methods may be combined.

Although in embodiments described above the strength or peak-to-peakamplitude of the bipolar signal is used, those skilled in the art willappreciate that other metrics may be used such as for example a simplepeak (maximum), a p-norm, area under the bipolar signal, etc.

As will be appreciated, embodiments described above can be performed inrealtime or off-line using images stored in memory.

Although the thermoacoustic imaging system is described as comprising anRF source configured to generate short pulses of RF electromagneticradiation, those skilled in the art will appreciate that in otherembodiments the thermoacoustic imaging system may comprise a visiblelight source or an infrared radiation source with a wavelengths between400 nm and 10 μm and a pulse duration between 10 picoseconds and 10microseconds.

Although in embodiments described above the thermoacoustic imagingsystem and the ultrasound imaging system are described as using one ormore ultrasound transducer arrays, those skilled in the art willappreciate that the alternatives are available. For example, a singletransducer element, an ultrasound transducer array having a linear orcurved one-dimensional array, or a two-dimensional ultrasound transducerarray may be used. In addition, a gel-like material or water capsule maybe used to interface the one or more ultrasound transducer arrays withthe region of interest.

Although in embodiments described above, the temperature is estimatedusing thermoacoustic data obtained of a single region of interest, thoseskilled in the art will appreciate that multiple regions of interest maybe analyzed and combined.

Although in embodiments described above blood vessels are described asbeing identified manually by an operator, those skilled in the art willappreciate that blood vessels may be identified in other ways. Forexample, in another embodiment automatic or semi-automatic algorithmsmay be used to identify one or more blood vessels. In other embodiments,Doppler imaging methods may be used to identify blood vessels.

Those skilled in the art will appreciate that the above-describedultrasound image data and thermoacoustic data may be one-dimensional,two-dimensional or three-dimensional. In embodiments, the ultrasoundimage data may be in a different dimension than the thermoacoustic data.For example, ultrasound image data may be two-dimensional and thethermoacoustic data may be one-dimensional. Further, different fields ofview may be used.

In another embodiment, different types or models of transducer arraysmay be used with the thermoacoustic and ultrasound imaging systems. Inthis embodiment, a transform may be used to map a thermoacousticabsorption image to the ultrasound image. In another embodiment, in theevent that knowledge of transducer array geometry is not readilyavailable, the thermoacoustic absorption image may be mapped to theultrasound image using phantom reference points. In this embodiment, atransform may be used to map known phantom reference points from thethermoacoustic absorption image to the phantom reference points on theultrasound image.

Although the ultrasound imaging system is described as using B-modeultrasound imaging techniques, other techniques may be used such as forexample power Doppler images, continuous wave Doppler images, etc.

Those skilled in the art will appreciate that other objects of interestmay be evaluated and other references may be used such as for examplethe heart, kidney(s), lung, esophagus, thymus, breast, prostate, brain,muscle, nervous tissue, epithelial tissue, bladder, gallbladder,intestine, liver, pancreas, spleen, stomach, testes, ovaries, uterus,skin and adipose tissues.

Although in embodiments described above thermoacoustic data is obtainedof the region of interest, those skilled in the art will appreciate thatthermoacoustic data may be obtained for an area larger than the regionof interest.

Using the foregoing specification, the invention may be implemented as amachine, process, or article of manufacture by using standardprogramming and/or engineering techniques to produce programmingsoftware, firmware, hardware or any combination thereof.

Any resulting program(s), having computer-readable instructions, may bestored within one or more computer-usable media such as memory devicesor transmitting devices, thereby making a computer program product orarticle of manufacture according to the invention. As such,functionality may be imparted on a physical device as a computer programexistent as instructions on any computer-readable medium such as on anymemory device or in any transmitting device, that are to be executed bya processor.

Examples of memory devices include, hard disk drives, diskettes, opticaldisks, magnetic tape, semiconductor memories such as FLASH, RAM, ROM,PROMS, and the like. Examples of networks include, but are not limitedto, the Internet, intranets, telephone/modem-based networkcommunication, hard-wired/cabled communication network, cellularcommunication, radio wave communication, satellite communication, andother stationary or mobile network systems/communication links.

A machine embodying the invention may involve one or more processingsystems including, for example, computer processing unit (CPU) orprocessor, memory/storage devices, communication links,communication/transmitting devices, servers, I/O devices, or anysubcomponents or individual parts of one or more processing systems,including software, firmware, hardware, or any, combination orsubcombination thereof, which embody the invention as set forth in theclaims.

Using the description provided herein, those skilled in the art will bereadily able to combine software created as described with appropriateor special purpose computer hardware to create a computer system and/orcomputer subcomponents embodying the invention, and to create a computersystem and/or computer subcomponents for carrying out the method of theinvention.

Although embodiments have been described above with reference to theaccompanying drawings, those of skill in the art will appreciate thatvariations and modifications may be made without departing from thescope thereof as defined by the appended claims.

What is claimed is:
 1. A method for estimating tissue temperature, themethod comprising: (i) directing, using a radio frequency (RF)applicator, one or more RF energy pulses into the tissue region ofinterest, the tissue region of interest comprising an object of interestand at least one reference that are separated by at least one boundary;(ii) detecting, using an acoustic receiver, at least one bipolaracoustic, signal generated in the region of interest in response to theRF energy, pulses and processing the at least one bipolar acousticsignal to determine a peak-to-peak amplitude thereof; and (iii)calculating a temperature at the at least one boundary using thepeak-to-peak amplitude of the at least one bipolar acoustic signal. 2.The method of claim 1, further comprising estimating a temperature ofthe object of interest based on the calculated temperature at the atleast one boundary.
 3. The method of claim 1, further comprisingdetecting, using the acoustic receiver, at least one other bipolaracoustic signal generated by a second object of interest in response tothe RF energy pulses and processing the at least one other bipolaracoustic signal to determine a peak-to-peak amplitude thereof.
 4. Themethod of claim 3, comprising comparing the peak-to-peak amplitude ofthe bipolar acoustic signal, the calculated temperature at the at leastone boundary and the peak-to-peak amplitude of the other bipolaracoustic signal to estimate a temperature of the second object ofinterest.
 5. The method of claim 1, wherein the boundary is at alocation between at least two different types of tissue.
 6. The methodof claim 5, wherein the two different types of tissue are one of: muscleand fat; a blood vessel and fat; and liver tissue and kidney tissue. 7.The method of claim 1, further comprising repeating steps (i) to (iii)during treatment to continuously monitor tissue temperature.
 8. A systemfor determining tissue temperature within a region of interestcomprising an object of interest and a reference that are separated byat least one boundary, the system comprising: a thermoacoustic imagingsystem comprising an adjustable radio frequency (RF) applicatorconfigured to emit RF energy pulses into the tissue region of interestand heat tissue therein and an acoustic receiver configured to receivebipolar acoustic signals generated in response to heating of tissue inthe region of interest; and one or more processors configured to:process at least one received bipolar acoustic signal generated in theregion of interest in response to the RF energy pulses to determine apeak-to-peak amplitude thereof; and calculating a temperature at the atleast one boundary using the peak-to-peak amplitude of the at least onebipolar acoustic signal.
 9. The system of claim 8, wherein the one ormore processors are further configured to: estimate a temperature of theobject of interest based on the calculated temperature at the at leastone boundary.
 10. The system of claim 9, wherein the one or moreprocessors are further configured to: detect, using the acousticreceiver, at least one other bipolar acoustic signal generated by asecond object of interest in response to the RF energy pulses andprocess the at least one other bipolar acoustic signal to determine apeak-to-peak amplitude thereof.
 11. The system of claim 10, wherein theone or more processors are further configured to: compare thepeak-to-peak amplitude of the bipolar acoustic signal, the calculatedtemperature at the at least one boundary and the peak-to-peak amplitudeof the other bipolar acoustic signal to estimate a temperature of thesecond object of interest.
 12. A method of monitoring tissuetemperature, the method comprising: (i) directing, using a radiofrequency (RF) applicator, one or more RF energy pulses into the tissueregion of interest, the tissue region of interest comprising an objectof interest and at least one reference that are separated by at leastone boundary; (ii) detecting, using an acoustic receiver, at least onebipolar acoustic signal generated in the region of interest in responseto the RF energy pulses and processing the at least one bipolar acousticsignal to determine a peak-to-peak amplitude thereof; (iii) calculatinga temperature at the at least one boundary using the peak-to-peakamplitude of the at least one bipolar acoustic signal; and (iv)repeating the directing, detecting and calculating to monitor thetemperature at the at least one boundary.
 13. The method of claim 12,further comprising: estimating a temperature of the object of interestbased on the calculated temperature at the at least one boundary. 14.The method of claim 12, further comprising: detecting, using theacoustic receiver, at least one other bipolar acoustic signal generatedby a second object of interest in response to the RF energy pulses andprocessing the at least one other bipolar acoustic signal to determine apeak-to-peak amplitude thereof.
 15. The method of claim 14, comprisingcomparing the peak-to-peak amplitude of the bipolar acoustic signal, thecalculated temperature at the at least one boundary and the peak-to-peakamplitude of the other bipolar acoustic signal to estimate a temperatureof the second object of interest.